Some editors don't show this - timeline, graph editor (more TODO's then blockers).

Currently headers and properties tabs have no menu access to add it back.

It's unclear if this design proposed to remove all or only some of these widgets, we could remove all, then make sure there are ways to add them back, however this is likely to be less discoverable - eg: Someone might might a header by accident, then not know to RMB over a blank area to toggle it back on.

The down side to removing only some of these is we have inconsistent ways to access regions.